1. Field of the Invention
The invention relates to the field of catalytic hydrocarbon gas reformers, and to an improved catalyst support material and catalyst support configuration for use in a catalytic hydrocarbon gas reformer. The invention is particularly useful in high temperature, solid oxide electrolyte electrochemical generators for electrical power generation. More particularly, one aspect of the invention is directed to a reforming catalyst support comprising a porous, non-rigid fibrous material having improved dimensional stability during prolonged operation, and another aspect of the invention is directed to a reforming catalyst support mounted in a configuration defining discrete flow paths along the length of the catalyst support, for improved heat transfer rate and resistance to pressure drop across the reformable hydrocarbon gas flow paths.
Natural gases comprising methane, ethane, propane, butane and/or nitrogen and the like, vaporized petroleum fractions such as naphtha and the like, and also alcohols, for example ethyl alcohol and the like, are appropriate fuels for electrochemical reactions, and can be consumed in an electrochemical generator apparatus for generating electrical power, for example, a high temperature, solid oxide electrochemical fuel cell generator. However, the direct use of hydrocarbon fuels for generating electrical power can cause carbon deposition or the formation of soot on the electrochemical fuel cells of the generator and other generator components, at least partly from hydrocarbon cracking. Deposition of carbon on the electrochemical generator components, for example, insulation boards, fuel distribution boards, support blocks, partition boards and fuel cells, reduces the efficiency of the electrochemical generator, inter alia, by blocking transport paths, providing electrical short-circuit paths and reducing insulation effects.
To reduce carbon deposition, it is known to reform the hydrocarbon feed fuel gas entering the fuel cell chamber of an electrochemical generator apparatus into simpler molecules, especially into carbon monoxide (CO) and hydrogen (H.sub.2), through the use of a reforming catalyst. Hydrocarbon reforming is therefore used to provide low carbonizing fuels for the electrochemical cells. It is also known that the presence of water vapor (H.sub.2 O.sub.(g)) and/or carbon dioxide (CO.sub.2) and a reforming catalyst allows for the direct conversion of gaseous hydrocarbons, such as natural gas, to CO and H.sub.2 by an endothermic reforming reaction, i.e., requiring a supply of heat. The reforming reaction is best performed at a temperature of about 900.degree. C.
The reformed hydrocarbon fuel is then combined, for example, in an electrochemical generator apparatus, along with an oxidant such as air, to produce heat and electric power. Since the reforming reaction is endothermic, additional thermal energy must be supplied, e.g., by direct combustion or by heat transfer through the walls of a heat exchanger, such as in a steam-air or air-air heat exchanger. Typically, the heat required for the reforming reaction in an electrochemical generator apparatus is derived from a significant fraction of the excess heat that results from operation of the electrochemical generator.
High temperature, solid oxide electrolyte fuel cells and multi-cell generators and configurations designed for converting chemical energy into direct current electrical energy, typically in the temperature range of from 600.degree. C. to 1200.degree. C., are well known and taught, for example, in U.S. Pat. Nos. 4,395,468 (Isenberg) and 4,490,444 (Isenberg). A multi-cell generator generally contains a plurality of parallel elongated, electrically interconnected, tubular, electrochemical fuel cells, each fuel cell having an exterior fuel electrode, an interior air electrode, a solid oxide electrolyte sandwiched between the electrodes, and means for entry of a gaseous oxidant and a gaseous hydrocarbon feed fuel. A previously reformed hydrocarbon feed fuel, i.e., converted to H.sub.2 and CO, is fed into the generator at one end and flows parallel to the exterior of a fuel electrode surface that is elongated along an axis. The fuel is oxidized by an oxidant, such as oxygen or air, which is fed into the generator at another end and parallel to the interior of the air electrode surface that is elongated along an axis. Direct current electrical energy is generated. Spent fuel is combusted with spent oxidant in a separate combustion chamber and is vented from the generator as a hot combusted exhaust gas.
In the known high temperature, solid oxide electrolyte fuel cells and multi-cell generators, the hydrocarbon feed fuel gas, such as natural gas, is generally mixed with water vapor and/or carbon dioxide, typically supplied from a recirculated spent fuel gas (unoxidized) typically containing H.sub.2 O and CO.sub.2, and is reformed as an initial step, i.e., converted to H.sub.2 and CO, through the use of a reforming catalyst, typically nickel or platinum, supported on a catalyst support material, typically rigid and highly sintered alumina pellets. Reforming the hydrocarbon feed fuel can be performed either inside or outside the electrochemical generator. However, hydrocarbon reforming outside of the electrochemical generator is less desirable in that heat energy is lost at the reformer and at the connecting conduits, making such a system more expensive and complicated than one with an internal reformer. Moreover, the hydrocarbon reforming reaction is performed at a temperature close to that of the electrochemical fuel cell operation. Accordingly, reforming efficiency is best where the reformer is inside the electrochemical generator and the largest possible fraction of excess heat that results from the fuel cell generator operation can be usefully applied.
U.S. Pat. No. 4,729,931 (Grimble) discloses a fuel cell generator having an internal catalytic hydrocarbon reformer where hot spent fuel gas containing H.sub.2 O and CO.sub.2 is recirculated and drawn into fresh hydrocarbon feed fuel at an ejector nozzle, and the reformable gaseous mixture is then fed through an internal hydrocarbon reforming chamber containing a packed reforming catalyst bed or packed column of finely divided Ni and Pt, disposed alongside the length of the fuel cell chamber. After flowing through the packed bed at about 900.degree. C., the reformable gaseous mixture yields a reformed fuel gas, namely H.sub.2 and CO, which is ultimately fed across the fuel electrodes in the fuel cell chamber. The use, however, of not easily monitored or controlled amounts of recirculated spent fuel gas as a source of H.sub.2 O and/or CO.sub.2 combined with fresh hydrocarbon feed fuel for the reforming reaction has a potential to result in several problems due to carbon deposition on the reforming catalyst during hydrocarbon reforming and other also on generator components. Carbon deposition on the internally located reforming catalyst and catalyst support structure can result in blocked flow paths across a catalyst bed, thereby increasing the pressure drop across the bed. It can also result in increased internal stresses in catalyst support structures which are conventionally porous, rigid, sintered, alumina pellets impregnated with a reforming catalyst, thereby causing pulverization and cracking of the catalyst support structure and reducing its reforming efficiency.
Carbon deposition on a hydrocarbon reforming catalyst surface is thought to result from insufficient adsorption of H.sub.2 O and/or CO.sub.2 on the reforming catalyst surface, i.e., insufficient presence of the oxygen species. Reduced gasification of carbon from the adsorbed hydrocarbon feed gas, and hydrocarbon cracking, are the results. The oxygen species is needed in sufficient quantity to react with the adsorbed carbon species to form carbon monoxide. Without oxygen, carbon is formed on the reforming catalyst and on other components of the electrochemical generator. This resulting deposited carbon is encapsulating in nature and is resistant to oxidation by H.sub.2 O present in the reforming atmosphere.
There have been attempts made to reduce carbon deposition on the hydrocarbon reforming catalyst and other electrochemical fuel cell generator components. In order to reduce carbon deposition on the reforming catalyst and reforming catalyst support structure, it is known to reform hydrocarbon feed fuel gas in an excess of water vapor and/or carbon dioxide in the presence of reforming catalyst.
U.S. Pat. No. 5,143,800 (George et al.) discloses a high temperature, solid oxide electrolyte fuel cell generator having an internal catalytic hydrocarbon reformer where spent fuel containing H.sub.2 O and CO.sub.2 is recirculated and aspirated into fresh feed hydrocarbon fuel at a circulation or mixing nozzle prior to entering the reforming chamber, and characterized in that the fresh feed inlet has a by-pass channel into the spent recirculated fuel channel having valving to control the spent fuel inclusion in the fresh hydrocarbon feed fuel prior to entering a reforming chamber containing a nickel catalyst. Additional spent fuel is combined with spent oxidant in a combustion chamber to form combusted exhaust gas that is circulated to heat the reforming chamber and other components of the fuel cell. The valve adjusted combination of spent fuel with fresh feed fuel attempts to prevent carbon deposition and soot formation within the reforming catalyst and reforming catalyst support structure and other fuel cell generator components.
Other attempts have been made to reduce carbon deposition on the hydrocarbon reforming catalyst and the reforming catalyst support structure. U.S. Pat. No. 5,169,730 (Reichner et al.) discloses a high temperature, solid oxide fuel cell having an internal catalytic hydrocarbon reformer where the recirculated spent fuel is cooled through heat transfer operations with other components of the fuel cell generator to a temperature of below 400.degree. C. prior to entering the nozzle or ejector located at a low temperature exterior position to the main body of the generator, and then mixing with the fresh hydrocarbon feed fuel to avoid hydrocarbon cracking at the nozzle and deactivation or poisoning of the reforming catalyst.
U.S. Pat. No. 4,898,792 (Singh et al.) discloses a high temperature, solid oxide electrolyte fuel cell generator having porous, fuel conditioner boards used to distribute a hydrocarbon fuel over the fuel cells and also to act in a hydrocarbon reforming capacity. In Singh et al., the reforming catalyst material used to reduce carbon formation includes a porous, rigid pressed or sintered felt of powder or fiber alumina as It catalyst support structure impregnated or treated with a reforming catalyst including catalytic Ni and also metal salts, the salts including nitrates, formates and acetates, and metal oxides, and the metals being selected from the group of Mg, Ca-Al, Sr-Al, Ce, Ba and mixtures thereof. It is known that metal oxides are effective in readily adsorbing gaseous H.sub.2 O.
In all prior designs, however, during long term reforming operation on hydrocarbon fuels, there remains the possibility of performance degradation of the reforming catalyst and reforming catalyst support structure, and also of other components of a fuel cell generator. Although the operation of a reformer, for example, in an electrochemical generator, is intended to take place in a relatively carbon deposition free operating range, prolonged operation could result in carbon formation on the catalyst and catalyst support due to occasional unavoidable variation from nominal operating parameters, such as, for example, a change in O:C ratios or a change in temperature of the reformer feed gas mixture.
Commercial reforming catalyst materials presently in use for hydrocarbon reforming in high temperature, solid oxide fuel cells typically include a catalyst carrier or support structure, active catalyst deposited or impregnated on the support structure surfaces, and optionally, other promoters. The catalyst support is typically a porous material having high total surface areas (internal and external) to provide high concentrations of active sites per unit weight of catalyst. The catalyst support is also typically a rigid material which is made to withstand high pressure operating conditions, i.e., mainly a carryover from the petrochemical industry, even though high pressure designs are generally not needed when used for hydrocarbon reforming in a high temperature, solid oxide fuel cell generator applications. The commercial reforming catalyst material typically used in high temperature, solid oxide fuel cell generators consists of a porous, rigid, support catalyst made from sintered and/or pressed powdered alumina (Al.sub.2 O.sub.3), that is impregnated with catalytic Ni and possibly MgO, typically in the form of pellets.
However, these commercial catalyst materials, including a porous, rigid, sintered alumina reforming catalyst support structures doped with catalyst, are prone to mechanical breakdown, thought to result in part from stresses generated in the rigid, sintered body by carbon formation on the reforming catalyst material during the reforming operation. The mechanical degradation of the reforming catalyst material, particularly the reforming catalyst support structure reduces the life of the catalyst material and, consequently, degrades the generator electrical output when used in connection with an electrochemical generator. Upon prolonged operation of the reformer, for example, in the electrochemical generator, the reforming catalyst material including the catalyst support and the catalyst deposited thereon are subject to mechanical disintegration, fracturing, dusting and/or pulverization, during carbon formation which can lead to a pressure buildup across the reformer bed, and, consequently, degradation of the generator electrical output. Moreover, the removal of the carbon, once formed, if needed to regenerate the surface activity of the catalyst by, for example, oxidation, is difficult. Them is a need to provide a reforming catalyst material including the catalyst support structure that is not subject to mechanical breakdown and dimensional instability during carbon deposition to provide prolonged catalyst operation, even at lower O:C ratios.
Moreover, commercial reforming catalyst materials typically used in high temperature, solid oxide fuel cell generators typically include catalyst support structures in the form of pellets which are packed in a tubular internal reforming chamber. As described above, the catalyst pellets are typically made from a porous, rigid, sintered alumina support structures which are doped with Ni and possibly MgO. These catalyst pellets can be configured in various shapes, such as spherical, oblate spheroid, annular ("Raschig rings") and wagon wheel shapes. The more complex shapes have relatively greater surface area than simple shapes (e.g., spheres), but complex shapes have drawbacks with respect to flow resistance and thermal conductivity through the catalyst bed as well as mechanical disintegration problems.
These catalyst pellets are further typically contained in a packed arrangement within an elongated reformer tube inside the fuel cell generator through which the reformable gas mixture stream is passed. These tightly packed catalyst pellets, however, resist gas flow and result in a substantial pressure drop through the catalyst bed. A low pressure drop of the reformable gas mixture stream is desirable through the catalyst bed, but is difficult to achieve in a bed comprised of such catalyst pellets. The pellets have an adverse impact on the reformable feed gas pumping pressure in the catalytic reformer.
In addition, whereas the reforming reactions are endothermic, the pellets detract from heat transfer from the reformer tube wall toward the center of the catalyst pellet bed. The pellets thus adversely affect the efficiency of the reforming reaction. To compensate, the reformer tube size must be reduced and elongated to provide a smaller cross section, and the overall compactness of the reformer suffers.
Typical hydrocarbon reformer designs consist of a plurality of long, thin tubes filled with these catalyst pellets. Such a reformer design is used to achieve high heat transfer rate while maintaining a long gas flow path over a large area of active catalyst. However, this configuration is not space or volume efficient. Moreover, it results in a relatively high pressure drop of the reformer gas stream through the catalyst bed. Some proposed hydrocarbon reformer applications are extremely limited in available space allocation and also in pumping pressure available to drive the reformer gas through the catalyst bed. An example is an internal reformer for a high temperature, solid oxide fuel cell recirculation generator incorporating an ejector or nozzle as the gas stream motive element. There is a need for a more optimal configuration of the reforming chamber and the reforming catalyst material contained therein including the catalyst support structure and catalyst deposited thereon, to improve heat transfer rates and resistance to pressure drops.
It would be advantageous for catalytic hydrocarbon reformers, especially in an electrochemical fuel cell generator apparatus, to contain a reforming catalyst material having a catalyst support structure impregnated with catalyst that is not prone to mechanical or dimensional breakdown due to carbon formation, improves gas stream pressure drop through the catalyst bed, and enables a high heat flux to pass from the catalyst containment wall to the reformable gas stream. According to one aspect of the present invention, a catalyst material is provided including a porous, non-rigid catalyst support material impregnated with a reforming catalyst. The non-rigid, catalyst support is compressible and improves the stability of the catalyst support against pulverization. Moreover, even in the event of generation operation where carbon formation may occasionally become possible, the non-rigid catalyst support of the invention provides structural stability to the catalyst material without pulverization of the catalyst support or the catalyst. According to another aspect of the present invention, a catalyst material is provided including a catalyst support configuration elongated in the direction of reformable gas flow having discrete flow paths or passageways along the catalyst support body to define a reformable gas mixture flow channel or channels therein which provide passageways for the reformable gas mixture at lower pressure drops and heat transfer rates. The catalyst support configuration, therefore, defines discrete passageways along its length for substantial portions of the reformable gas mixture, improving heat transfer properties and reducing the pressure drop and pumping requirements across the catalyst bed.